Data from: Migration of black terns Chlidonias niger and common terns Sterna hirundo between South Sweden and the Atlantic coast of Africa

Light-level geolocators were used to record the annual migration cycles of
black terns Chlidonias niger (9 inds, 11 journeys) and common terns Sterna
hirundo (7 inds, 11 journeys) breeding in southernmost Sweden. The black
terns used two different winter (Oct – Mar) regions along the Atlantic
coast of Africa, either north of the equator between Senegal and Liberia
(3 inds) or south of the equator between Gabon and northern Namibia (5
inds). All the common terns travelled to winter quarters south of the
equator, mainly along the coasts of Namibia and South Africa. One juvenile
common tern was tracked during the first twenty months of its life. This
bird spent its first winter at South Africa, after which it migrated north
of the equator to over-summer as one-year old non-breeder in tropical
waters off Ghana, after which it returned to South Africa for its second
winter. This record demonstrates that one-year old terns may undertake
extensive intra-African migration to distant over-summering areas.
Comparing geolocator results from Swedish and Dutch black tern populations
indicate that they have similar migration habits, with a possible tendency
of relatively more individuals migrating south of the equator in the more
northerly Swedish population (leap-frog migration). Comparing geolocator
and ringing results among common tern populations indicates a fascinating
and complex pattern of scale-dependent geographic segregation and
intermixing along the coasts of Africa.